The invention relates to reel sensors and start/finish end detecting devices for detecting a start/finish end of a video tape.
If a video tape is left to travel continuously when the finish end thereof nears, a leader portion of the video tape comes in contact with the rotary head, so that the rotary head is likely to be damaged. Further, if the video tape is wound continuously even when the finish end thereof has appeared, the video tape may likely to be torn. To overcome these problems, a video cassette deck is equipped with a start/finish end detecting device.
Further, in order to detect the type of recording time, remaining time, and the like of a video tape or in order to minimize chances of the video tape being damaged by detecting defective winding based on a ratio in the number of revolutions between a reel stand on the supply side and a reel stand on the wind side, a reel sensor is additionally provided.
FIG. 9 is a sectional view outlining a start/finish end detecting device and a reel sensor, which are conventional examples.
More specifically, the start/finish end detecting device includes: a light-emitting element 64 that radiates an infrared ray upward; a light-emitting side prism 65 that reflects the infrared ray radiated from the light-emitting element 64 in horizontal directions and radiates the infrared rays to incident light windows 62a, 62b formed in a video cassette 61; light-receiving prisms 66a, 66b that reflects the infrared rays radiated from emergent light windows 63a, 63b downward; and light-receiving elements 67a, 67b that receive the infrared rays reflected by the light-receiving prisms 66a, 66b.
That is, when the magnetic substance applied portion of a video tape is travelling, a video tape 69a, 69b that crosses the optical paths 68a, 68b of the infrared rays is opaque. For this reason, the infrared rays radiated from the light-emitting element 64 are shielded by the video tape 69a, 69b, thereby not reaching the light-receiving elements 67a, 67b. On the other hand, when the leader portion appears after the magnetic substance applied portion has ended, the video tape 69a, 69b become transparent, which in turn allows the infrared rays radiated from the light-emitting element 64 to be received by the light-receiving elements 67a, 67b. As a result, start/finish ends of the video tapes can be detected by the outputs of the light-receiving elements 67a, 67b.
The reel sensor includes: a plurality of light-shielding blades 72a, 72b formed on the lower surfaces of reel stands 71a, 71b; and photo-interrupters 73a, 73b that detect the passing of the light-shielding blades 72a, 72b.
That is, when the reel stands 71a, 71b are rotating, the light-shielding blades 72a, 72b cross the slits of the photo-interrupters 73a, 73b. As a result, pulses are generated as the outputs of the photo-interrupters 73a, 73b every time the light-shielding blades 72a, 72b pass the photo-interrupters. The pulses are generated at a time interval corresponding to the rotational speeds of the reel stands 71a, 71b. As a result, the rotational speed of each of the reel stands 71a, 71b is calculated based on the time interval at which the pulses are generated. Further, it if no pulses appear at the outputs of the photo-interrupters 73a, 73b although pulses must appear, then it means that the reel stands 71a, 71b are defectively rotating, so that occurrence of such an abnormality as defective winding or torn tape is indicated.
However, in the aforementioned construction, the start/finish end detecting device requires a single light-emitting element 64 and two light-receiving elements 67a, 67b. Further, the reel sensor requires that the photo-interrupters 73a, 73b be provided for the respective reel stands 71a, 71b. That is, each of the reel stands 71a, 71b requires a single light-emitting element and a single light-receiving element.
That is, if both the start/finish ends of the video tape and the rotational speeds of the reel stands 71a, 71b are to be detected, three (3) light-emitting elements and four (4) light-receiving elements must be provided. On the other hand, the light-emitting elements and the light-receiving elements are energy converting elements from light to electricity and vice versa, and therefore expensive. For this reason, the construction requiring the three light-emitting elements and the four light-receiving elements has increased the manufacturing cost.